An industrial robot is programmed to carry out work or a work cycle along an operating path. In order to program or teach the robot the work cycle, the robot is manipulated to positions along the desired operating path. These positions are stored as instructions in a memory in the control unit. Other information, such as desired robot movement velocity, may also be stored in the memory. During operation of the robot, the program instructions are executed, thereby making the robot operate as desired.
A robot is operated in at least two modes: automatic and manual. By switching a mode selector key on a control panel on the control unit, the choice of mode, in which the robot shall operate, is made. When the robot is in the manual mode, the TPU has the exclusive right to operate the robot and thus no other operating control device or control unit is allowed to control the robot.
An industrial robot is usually placed in robot cells in order to facilitate the automation of a complex series of actions under safe conditions. When one of the robots in a robot cell is to be operated under manual control using a TPU, it is important that the TPU is communicating with and controlling the right robot. Thus, in order to accomplish a safe control of an industrial robot, it is important to have absolute confidence in the control of the robot. The operator must be fully aware of the current mode of the connection between the control unit and the TPU.
In an industrial robot system where the TPU is connected to the control unit by a cable, the operator can easily be aware of the current mode of said connection. The cable is preferably shielded and no other communication systems can interfere with the two connected units. Under normal conditions the control unit periodically transmits data to the TPU. Upon receiving this data the TPU responds by transmitting a response signal back to the control unit. Thus, the control unit asks the TPU if this is in operation. The control unit judges from said response signal if the TPU is in normal condition. If the emergency switch has been activated, the response signal carries that information and the control unit executes the emergency stop. The length of the cable defines the maximum distance between a TPU cable connected to an industrial robot and the control unit.
In an abnormal situation, e.g. when the operating unit is disconnected or the cable is cut off, the TPU does not transmit a responding signal. The absence of a responding signal is a sign to the control unit that there is an emergency stop situation and the control unit will activate the emergency stop.
However, the TPU cable ends up entangled on the floor in industrial robot systems with several robots and hence several cables. The cables are often lying on the floor and there is an obvious risk of damage by a vehicle running over. There is also a risk of cable entanglement by which an operator will have difficulties in knowing which TPU belongs to which robot. Furthermore, a TPU connected by cable to a control unit limits the degree of freedom that an operator needs to perform an effective operation with the robot.
In accordance with the conditions mentioned above, the development of industrial robot systems is in one way leading to a wireless TPU for the robot control. A wireless connection eliminates most of the negative consequences of using a cable connection. On the other hand, a wireless connection introduces some problems of its own. Most notable is the problem of ensuring that the TPU is associated with the correct robot and the problem of verifying that the wireless connection works correctly.
In both cases, there is a risk of personal injury if the operator believes that he is operating and controlling a certain robot but instead is out of contact with this particular robot. Thus, there is a strong need to ensure connection between the robot and the TPU when using a TPU not physically connected to the robot. In other words, it is of great importance that the operator is immediately made aware of a sudden communication loss of the TPU. Since the operator usually is concentrated on the robot and there may be a great deal of ambient noise, the usual warnings (blinking light, buzzer) are ineffective and not reliable.
One security arrangement in an industrial robot system with a robot working in a cell is designed as follows. If something abnormal happens in the cell, an operation stop is activated and the operating robot is stopped and remains standing still. After reestablishment of a normal condition in the cell, the robot will be restarted. In this situation, a stopped robot does not disturb the rest of the robot system. Then, there is a higher degree of security stop and that is the emergency stop for stopping the whole robot system. This is activated when something extraordinary happens somewhere in the robot system. Closing down the whole robot system is undesirable since it necessitates a great deal of work for the operator to return the system to operation conditions.
From the Japanese patent application 11-73201, a wireless control system is previously known. The object of the invention is to provide a communication system wherein the specific communication of emergency stop control can be carried out by wireless means in a manner equivalent to cable communication. The solution of the invention is in the design of the system with data exchange by means of wireless or optical communication between a first device and a second device. The first device comprises a transmitting part and a receiving part. The second device comprises a receiving part and a transmitting part, which returns arbitrary data in response to data received from said first device at a normal time, and stops said response at an abnormal condition. The first device judges the condition to be abnormal by recognizing the received data to be arbitrary data. It also judges the condition to be normal by recognizing the received data to be specific data. Thus, specific communications such as emergency stop control is carried out by wireless means in a manner equivalent to the prior cable communication, and the system is made fail-safe.
A wireless connection is less predictable than a wired connection, and the communication between the TPU and the control unit may fail, be loosen or be broken by the system due to either distance, signal interference, radio shadow, current interruptions or battery failure, all of which will cause a communication loss. All different communication losses are denoted “a broken data link” and all different operating communications are denoted “a connected data link” in the following.
If a broken data link occurs, arrangements required by safety regulations will immediately stop the robot. These security arrangements are further programmed to measure the time for the robot standing still in accordance with the situation mentioned above. When the robot has been standing still for a predetermined time due to the broken data link, the emergency stop is activated and all activities in the cell will be stopped. The time between these events is chosen in order to comply with the safety regulations in force.
Despite necessity of security systems, a sudden shutdown of a robot system is a very frustrating situation for the operator. It requires time and energy for realizing the situation. It also takes a great deal of effort and time to reestablish the working condition in the robot system. This is a situation every operator is anxious to avoid, especially when there is no reason for the stop.
According to the conditions mentioned above, there is a need for an industrial robot system including a TPU, which system has the function of immediately making an operator aware of a broken data link. More precisely, there is a need for a TPU, which has the character of immediately indicating a broken data link to an operator carrying the TPU.